An unsophisticated user might enter their forum.bitcoin.org credentials into that prompt.

More generally, loading offsite images is an information leak (IP addresses of forum readers) and possibly even security risk (if any browser image-handling flaw would let the source site do more, such as redirect to some other site's XSSCSRF flaw, run JS, or in a worst-case, buffer-overflow for local code execution).

I suggest in our new security-conscious era, loading of offsite images as profile icons be disabled.

also links to external HTTP images break the lock icon in HTTPS connections

Bitcoin Core developer [PGP]Warning: For most, coin loss is a larger risk than coin theft. A disk can die any time. Regularly back up your wallet through File → Backup Wallet to an external storage or the (encrypted!) cloud. Use a separate offline wallet for storing larger amounts.